The XGC1 edge gyrokinetic code is used to study the width of the heat-flux to divertor plates in attached plasma condition. The flux-driven simulation is performed until an approximate power balance is achieved between the heat-flux across the steep pedestal pressure gradient and the heat-flux on the divertor plates. The simulation results compare well against the empirical scaling λ q ∝1/B P γ obtained from present tokamak devices, where λ q is the divertor heat-flux width mapped to the outboard midplane, γ=1.19 as found by T. Eich et al. [Nucl. Fusion 53 (2013) 093031], and B P is the magnitude of the poloidal magnetic field at the outboard midplane separatrix surface. This empirical scaling predicts λ q ≲1mm when extrapolated to ITER, which would require operation with very high separatrix densities (n sep /n Greenwald > 0.6) [Kukushkin, A. et al., Jour. Nucl. Mat. 438 (2013) S203] in the Q=10 scenario to achieve semi-detached plasma operation and high radiative fractions for acceptable divertor power fluxes. Using the same simulation code and technique, however, the projected λ q for ITER's model plasma is 5.9 mm, which could be suggesting that operation in the ITER Q=10 scenario with acceptable divertor power loads may be obtained over a wider range of plasma separatrix densities and radiative fractions. The physics reason behind this difference is, according to the XGC1 results, that while the ion magnetic drift contribution to the divertor heat-flux width is wider in the present tokamaks, the turbulent electron contribution is wider in ITER. Study will continue to verify further this important projection. A high current C-Mod discharge is found to be in a mixed regime: While the heat-flux width by the ion neoclassical magnetic drift is still wider than the turbulent electron heat-flux width, the heatflux magnitude is dominated by the narrower electron heat-flux.

Plasma profiles across the separatrix and scrape-off layer (SOL) in Alcator C-Mod are examined for a range of plasma densities, currents and magnetic fields in ohmic L-mode discharges and for a subset of conditions in ohmic H-mode discharges. In all plasmas, electron pressure gradient scale lengths (Lp e) exhibit a minimum value just outside the separatrix (i.e., in the near SOL), forming the base of a weak (strong) pedestal in L-mode (H-mode) plasmas. Over a wide range of conditions in ohmic L-mode discharges, Lp e at this location are found to track with a monotonic function of electron collision frequency, when this quantity is normalized according to the framework of electromagnetic fluid drift turbulence theory. Moreover, at fixed values of normalized collisionality (characterized as the 'diamagnetic parameter', d), electron pressure gradients in the near SOL increase with plasma current squared, holding the MHD ballooning parameter, MHD , unchanged. Thus, the state of the near SOL is restricted to a narrow region within this two-parameter phase-space. An implication is that cross-field heat and particle transport are strong functions of these parameters. Indeed, as d is decreased below ~0.3, cross-field heat convection increases sharply and competes with parallel heat conduction along open field lines, making high plasma density regions of MHDd space energetically inaccessible. These observations are consistent with the idea that the operational space of the edge plasma, including boundaries associated with the tokamak density limit, are controlled by electromagnetic fluid drift turbulence.

Ohmic energy confinement saturation is found to be closely related to core toroidal rotation reversals in Alcator C-Mod tokamak plasmas. Rotation reversals occur at a critical density, depending on the plasma current and toroidal magnetic field, which coincides with the density separating the linear Ohmic confinement regime from the saturated Ohmic confinement regime. The rotation is directed co-current at low density and abruptly changes direction to counter-current when the energy confinement saturates as the density is increased. Since there is a bifurcation in the direction of the rotation at this critical density, toroidal rotation reversal is a very sensitive indicator in the determination of the regime change. The reversal and confinement saturation results can be unified since these processes occur at a particular value of the collisionality.